For centuries, human beings have pondered this question. Medieval scholars speculated that other worlds must exist and that some would harbor other forms of life.

In our time, advances in science and technology have brought us to the threshold of finding an answer to this timeless question.

The recent discovery of numerous planets around stars other than the Sun confirms that our solar system is not unique. Indeed, these "extrasolar planets" appear to be common in our galactic neighborhood.

The extrasolar planets we have discovered thus far are giants, like Jupiter and Saturn. They are unlikely to support life as we know it. But some of these planetary systems might also contain smaller, terrestrial planets like Mars and Earth.

Over the next 15 years, NASA is embarking on a bold series of missions to find and characterize new worlds. These will be the most sensitive instruments ever built, capable of reaching beyond the bounds of our own solar system.

Artist's concept of the Terrestrial Planet Finder observatories

The Keck Interferometer will combine the light of the world's largest optical telescopes, extending our vision to new distances.

Using a technique known as interferometry, the Keck will study dust clouds around stars where planets may be forming. It may also provide the first direct images of giant planets outside our solar system.

SIM PlanetQuest, scheduled for launch within the next decade, will measure the distances and positions of stars with unprecedented accuracy. SIM's precision will allow us to detect evidence of planets just slightly larger than Earth.

Finally, the Terrestrial Planet Finder will build upon the legacy of all that have gone before it. With an imaging power 100 times greater than the Hubble Space Telescope, Terrestrial Planet Finder will send back the first photographs of nearby planetary systems.

Artist's concept of an Earth-like planet

We will analyze the atmospheres of these distant worlds, looking for carbon dioxide, water and ozone. The substantial presence of all three gasses would suggest that life is present.

Such a discovery would at last provide convincing evidence that we are not alone.